Dry-etching method

ABSTRACT

A dry-etching method for etching materials of the silicon group comprises: providing the material to be etched in a reaction chamber; supplying a mixed gas as the etching gas comprising carbon fluoride, oxygen and another gas wherein the other gas is a partially halogenated hydrocarbon; and thereafter subjecting the etching gas to high frequency electric current so as to make the mixed gas into a plasma whereby the material is etched.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a dry-etching method for making electronic parts such as semiconductor devices or the like, and especially to a dry-etching method for silicon nitride (Si₃ N₄).

2. Description of the Prior Art

In the conventional dry-etching method for silicon materials, especially for Si₃ N₄ film on SiO₂ film or Si₃ N₄ film masked by SiO₂ film, a mixed gas of carbon fluoride such as CF₄ which contains several percent of O₂ has been used. By making a high-frequency electric discharge in such etching gas, F radical reacts to Si and changes it into a volatile material, and etching process proceeds. Namely, an electron produced by the electric discharge reacts to CF₄ and changes it into an F radical, an F ion and an electron. The reaction proceeds by the following reaction as follows:

    CF.sub.4 +e→CF.sub.3 +F+e                           (1)

Further, CF₃ reacts with an O radical and number of the F radical increases. The reaction proceeds by the following reaction: ##EQU1## And the F radical reacts with Si, so the etching process proceeds as follows:

    Si+4F→SiF.sub.4                                     ( 3)

The conventional dry-etching method, however, has a problem in that it is impossible to selectively etch only Si₃ N₄ film, which is intended to be selectively without hardly etching the SiO₂ base film, the SiO₂ masking film or the photo-resist (hereinafter abridged as P.R) Namely, the combination Si and O in SiO₂ is comparatively stable and hardly separated by the impinging of F radical. However, it is considered that the combination is separated by the impinging O radical, and the separated and retained Si combines with the F radical, and so the etching process proceeds. The reaction proceeds as follows: ##EQU2## Therefore, in the use of mixed gas of carbon fluoride and O, the selection ratio of Si₃ N₄ /SiO₂ which is obtained is only 2 or 3 (which is defined as ratio of etching rate of Si₃ N₄ /etching rate of SiO₂).

As an alternative method, a case may be considered that the etching gas contains only carbon fluoride in the absence O. However, such case has another problem of C adhering onto the surface of Si₃ N₄ film with the resultant substantial ceasing of the etching process.

On the other hand, another method is considerable, such that a part of F of carbon fluoride is replaced by bromine to selectively etch the Si₃ N₄ film hardly etching the SiO₂ film. It is known that the selection ratio of Si₃ N₄ /SiO₂ increases by adding several percent of such bromic-replaced gas within the mixed gas of carbon fluoride and O₂ as mentioned above. The Br atom excited by plasma electric discharge is easy to combine with the O atom as follows:

    Br.sub.2 +O→BrO+Br                                  (5)

In other words, the Br atom has an effect to suppress the O atom which contributes to the etching process. And responding to the suppression of function of the O atom, the number of F radical decreases and the etching rate of Si₃ N₄ also decreases, and furthermore, the etching of SiO₂ is suppressed. As a result, the selection ratio of Si₃ N₄ /SiO₂ rises.

FIG. 1 shows the characteristic curves of the etching rate of Si₃ N₄ and the selection ratio of Si₃ N₄ /SiO₂ as a function of the concentration of CF₃ Br (volume percent, namely equal mol %) under the condition of mixing CF₃ Br gas into (CF₄ gas containing 5 Vol % of O₂). In this experiment, the Si₃ N₄ film and SiO₂ film accumulated on the surface of silicon substrate were etched by using the P.R pattern as a mask under the condition that the total quantity of gas flow was 50 SCCM (standard Cubic Centimeter per Minute, which means the quantity of flow in standard state of 273° K. and 760 mmHg.), the pressure of the gas was 0.4 Torr, and electric power of high frequency was 150 W (0.5 w/cm²). As shown in FIG. 1, responding to the increase of the concentration of CF₃ Br, the etching rate of Si₃ N₄ decreases, and moreover the etching of SiO₂ is suppressed, so the selection ratio of Si₃ N₄ /SiO₂ increases as shown by chain line curve in FIG. 1. And the characteristic curve of the concentration of CF₃ Br has a peak at the position over twelve percent. In this case, the etching rate of Si₃ N₄ was 960 angstrom per minute and the selection ratio of Si₃ N₄ /SiO₂ was 6.1 and Si₃ N₄ /P.R was 4.0. As mentioned above, such prior art is known that the mixing of CF₃ Br gas into CF₄ +5% O₂ makes the selection ratio of Si.sub. 3 N₄ /SiO₂ increase to some extent. But there is a strong demand for still higher selection ratio, with increasing the etching rate of Si₃ N₄.

SUMMARY OF THE INVENTION

Accordingly, the purpose of the present invention is to provide an improved dry-etching method for etching material of the Si group, without hardly etching SiO₂ mask or SiO₂ base or P.R mask. The method is especially suitable for dry-etching silicon nitride (Si₃ N₄) with a high selection ratio against P.R or SiO₂ film.

A dry-etching method for etching material such as silicon nitride in accordance with the present invention has steps of:

setting an etched material of silicon nitride into a reaction chamber,

supplying a mixed gas as an etching gas, which is made of carbon fluoride, oxygen and another gas having such a molecular structure that a part of hydrogen of hydrocarbon is replaced by halogen, into the reaction chamber, and

giving high frequency electric power so as to make the mixed gas as plasma state and to etch the etched material.

The material from the silicon group includes, for example, single crystalline silicon (used for making a semiconductor device), polycrystalline silicon, or silicon nitride.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the graph of characteristic curves of the etching rate of Si₃ N₄ and a selection ratio of Si₃ N₄ /SiO₂ and Si₃ N₄ /P.R as a function of concentration of CF₃ Br under the condition of mixing the CF₃ Br gas into the CF₄ +5% O₂ gas (CF₄ gas containing 5 Vol % of O₂ gas).

FIG. 2 is a graph of characteristic curves in accordance with the present invention of an etching rate of Si₃ N₄ and a selection ratio of Si₃ N₄ SiO₂ and Si₃ N₄ /P.R as a function of concentration of CH₃ Br under the condition of mixing the CH₃ Br gas into the CF₄ +10% O₂ gas (CF₄ gas containing 10 Vol % of O₂ gas).

FIG. 3 is a cross sectional view of an etching apparatus for use in the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is detailed description of the preferred embodiment of the present invention.

FIG. 2 shows a characteristic curves of an etching rate of Si₃ N₄ and a selection ratio of Si₃ N₄ /SiO₂ and Si₃ N₄ /P.R as a function of concentration of CH₃ Br under the condition of mixing CH₃ Br gas into CF₄ +10% O₂ gas (CF₄ gas containing 10 Vol % of O₂ gas).

FIG. 3 is a cross sectional view of an etching apparatus which is used in the embodiment in accordance with the present invention. The etching apparatus comprises a reaction chamber 1 which is to be filled with an etching gas kept in 0.4 Torr, an exhaust opening 2, a gas cylinder 3 filled with the etching gas of high pressure, a flow control valve 4, a power supplying electrode of high frequency of electric power 5, a grounded electrode 6, and an electric power source 7 for supplying high frequency electric power for the power supplying electrode 5. A substrate 8 which is to be etched by the etching gas is disposed on the surface of the power supplying electrode 5. In the embodiment of the present invention, the substrate 8 is Si wafer which has an Si₃ N₄ film or SiO₂ film on its surface. In this embodiment, etching process is made under the condition that the total quantity of gas flow is 50 SCCM, the pressure of gas is 0.4 Torr and electric power of high frequency is 150 W (0.5 w/cm²). As shown in FIG. 2, responding to the increase of the concentration of CH₃ Br, the etching rate of SiO₂ quickly decreases as the concentration of CH₃ Br increases, and as shown by the chain line at the position over six percent of the concentration of CH₃ Br, the etching rate of Si₃ N₄ is 1120 angstrom per minute and the selection ratio of Si₃ N₄ /SiO₂ is 13.5. And selection ratio of Si₃ N₄ /P.R is 6.7.

As mentioned in the prior art, mixing of CF₃ Br into the mixed gas of CF₄ +5% O₂, the selection ratio of Si₃ N₄ /SiO₂ increases. It is because Br reacts with O radical and suppresses the action of O radical which separates the combination of Si and O of SiO₂. In the present invention, using the mixed gas containing CH₃ Br, provides not only the above-mentioned Br but also H separated from CH₃ Br reacts as,

    2H+O→H.sub.2                                        (6)

by which the reaction of O radical is suppressed, so that the etching rates of SiO₂ and P.R rises and the selection ratio of Si₃ N₄ /SiO₂ or Si₃ N₄ /P.R sharply increase.

Accordingly, by using the dry-etching method in accordance with the present invention, namely by using the mixed gas of carbon fluoride, O₂ and the gas having such a molecular structure that a part of H of hydrocarbon is replaced by halogen, the material of Si group such as Si₃ N₄ can be etched with high selection ratio of such materials against SiO₂ mask, SiO₂ base or P.R mask as a result of such selective etching that the fluoride hardly etches the SiO₂ mask or base. In this embodiment, Br is used as halogen, but I has the same effect as Br. 

What is claimed is:
 1. A dry-etching method for etching silicon nitride having steps of:setting an etched material of said silicon nitride into a reaction chamber, supplying a mixed gas as an etching gas, which is made of carbon fluoride, oxygen and another gas having such a molecular structure that a part of hydrogen of hydro-carbon is replaced by bromine, into said reaction chamber, and giving high frequency electric power so as to make said mixed gas as plasma state and to etch said etched material.
 2. A dry-etching method in accordance with claim 1, whereinsaid etching gas is a mixed gas of CF₄, O₂ and CH₃ Br.
 3. The method according to claim 2, wherein in said method about four to about eight volume percent CH₃ Br is employed.
 4. The dry-etching method according to claim 2, wherein in said method about two to about twelve volume percent of CH₃ Br is employed.
 5. The method according to claim 2, wherein in said method about four volume percent to about ten volume percent of CH₃ Br is employed.
 6. A method for dry-etching comprising:providing a silicon nitride containing material to be etched into a reaction chamber; etching said material by contacting said material with the constituents of a plasma formed in a gas mixture which contains carbon floride, oxygen and a partially brominated or partially iodated hydrocarbon.
 7. The method according to claim 6, wherein in said method about two to about 12 volume percent of said partially brominated or said partially iodated hydrocarbon is employed.
 8. The method according to claim 6, wherein in said method about six volume percent of said partially brominated or said partially iodated hydrocarbon is employed.
 9. The method according to claim 6, wherein in said method about four to about eight volume percent of said partially brominated or said partially iodated hydrocarbon is employed. 